Worms are invertebrates from a variety of distantly related animal groups. They are primitive invertebrates with elongated bodies, a mouth, gut and anus. They are usually limbless although a number of species have many hair-like structures that help with movement. Worms include invertebrates from phyla such as Annelida (earthworms, polychaetes), Nematoda (roundworms), Nemertea (ribbonworms) and Platyhelminthes (flatworms).

Where do worms live?

Worms are most common below ground, both on land and in aquatic environments. Earthworms, the most well-known worms, live within the soil all over the world. Marine polychaetes are almost absent on land and in freshwater but are massively abundant in marine habitats. Other worms live in a variety of habitats such as beneath the soil or sea floor, within a self-made tube, and within seaweed, plants and animals.

How do they reproduce?

There is a huge variety of ways that worms are able to reproduce. In polychaetes alone there are at least 17 known methods of reproduction, including a range of both sexual and asexual methods. Sexual reproduction, which involves two individuals, is achieved with methods such as copulation (the method humans use), broadcast spawning (where eggs are released into water to be fertilized), and epitoky (a body segment filled with gametes that floats to the surface and then releases the eggs or sperm).

Worms are often very good at regenerating themselves and reproducing asexually. Many species are able to reproduce simply by splitting in half and producing a second identical individual, a process known as transverse fission. A second common method of asexual reproduction in worms is budding, where a worm grows a fully developed individual which then separates from its ‘mother’.

Body structure of worms

Worms have elongated bodies and are usually limbless. They have a very primitive head with just a mouth and simple sensory and nervous systems; no eyes, ears or nose. More ‘developed’ than jellyfish, corals and sponges; worms generally have three layers of tissue which allows for the formation of organs and a one-way digestive system so food can be moved through the body in one direction. In comparison, these more primitive animals take food in through the mouth, digest it and then release the waste back out through their mouth.

Interesting facts:

  • Most worms are less than 1 cm long but the world’s longest worm, a ribbon worm, can reach a massive length of up to 40 m, making it the longest animal on Earth.
  • A number of species of earthworms can grow longer the 60 cm (2 ft.) and the largest, the South African earthworms, can reach a massive length of over 6.5 m (22 ft.).

Last edited: 18 November 2015

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